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Stem Canker and Black Scurf

(Rhizoctonia solani)

The fungus causing stem canker and black scurf is present in nearly all soils because it has a wide host range, survives in plant debris, and as sclerotia is easily disseminated on tubers.
It grows over a wide range of temperatures. It causes considerable damage to emerging sprouts when conditions do not favor rapid emergence, such as cold and wet soil.

Symptoms of stem canker and black scurf

stem-canker-black-scurfLesions on sprout tips cause delayed emergence or failure to emerge.

Slightly sunken brown cankers of variable size and shape affect stolons and stems at or below the soil line.
Cankers may girdle stems and result in aerial tuber formation, plant wilt, and death. Girdled stolons may fail to produce tubers.
Hard, dark brown or black sclerotia (fungus-resting bodies) of irregular size and shape form on the tuber surface.
A white mycelial mat may develop on the stem base, but does little harm to the plant.

Management

Because sclerotia are long-lived in the soil, while growing potatoes only long rotations with cereals and grasses reduce disease incidence.
Shallow planting of well-sprouted tubers reduces the exposure time of sprouts in the soil.
This potatoes disease can be reduced by applications of soil fungicides such as PCNB (pentachloronitrobenzene) mixed into the planting band of soil.

Potatoes seed tuber treatment effectively reduces seed-borne inoculum when soils are not heavily infested.

Trichoderma and binucleate Rhizoctonia used as biological control agents reduce severity.

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Black Rot

(Rosellinia sp.)

Black rot, in higher tropical altitudes, may cause heavy potato yield losses in moist soils rich in organic matter (recently cleared land) and may affect other crop and weed hosts.

Symptoms of Black Rot

Black-rotDiseased potato plants are stunted and wilt. Underground organs develop a black rot partially covered by matted strands of grayish white mycelium.

The blackened potatoes tubers in cross section show a band of striate projections penetrating into the flesh. Often, only isolated potato plants are affected or the disease occurs in patches in the field. Only during lack of crop rotation while growing potatoes does the disease become a major problem.

Management

Removal or burning of brush and trees when land is cleared and subsequent crop rotation are recommended control measures.

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Stem Rot Potato disease

(Sclerotium rolfsii)

Stem rot, southern blight, or Sclerotium rot occurs worldwide and attacks a wide range of potatoes plants, but is a problem for potato only while growing under hot, moist conditions.

Symptoms of Stem Rot

Initially, daytime wilting and yellowing result when brownish lesions girdle the stem base just below the soil line.

A white mycelium grows on potatoes stems, tubers, or soil, often in fan-like mats, which produce small, initially white, but later brown, sclerotia, similar in appearance to mustard seed.

stem-rotIn rainy weather, the affected stem sloughs off, leaving only the vascular tissue of the xylem, which leads to stem collapse.
Potato tubers may rot in the field before harvest, in storage, or in transit. They first form a cheesy semi-firm decay, which is often invaded by soft rotting organisms. Seed tubers may decay before plant emergence.

Management

Sclerotia are long-lived and many crops are susceptible to S. rolfsii.
Control of stem rot is difficult when conditions favour disease.

Either plow deeply to bury affected plants and crop residues or remove them from the field and burn or bury them.
Avoid plant debris around potato stems.

The pathogen is highly aerobic; therefore, deep plowing and rotation with paddy rice are effective.
Harvest  potatoes during dry weather.
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